Drawing No. EH–TH–010 // Thermal Engineering & HVAC
Dehumidifier Sizing Calculator
Reviewed September 2026
Choose a simple appliance-sizing guide or estimate the actual moisture load from humidity, air exchange and internal sources.
01 // Space & moisture
02 // Recommended capacity
Worked example — why two methods disagree
A 6 × 5 m room, 2.5 m high (30 m², 75 m³), moderately damp, 22 °C, pulling 75% RH down to 50%, with 0.5 air changes per hour of 15 °C / 80% RH outside air and 2 L/day of internal moisture generation.
The guide method says 9.5–14.2 L/day. ENERGY STAR publishes a capacity range by floor area and dampness category. It is a lookup, not a calculation: 30 m² in the "moderately damp" band returns that range directly.
The psychrometric method says 3.6 L/day. It adds up the actual moisture to be removed:
One-time pull-down 0.37 L (the water held in 75 m³ of air, 75% → 50% RH)
Continuous load 2.3 L/day (infiltration + internal generation)
Safety margin 20%
Total 3.6 L/day
The two differ by roughly a factor of three, and neither is wrong.
The number that surprises people is the pull-down: only 0.37 litres. The air in a 75 m³ room simply does not hold much water — at 22 °C, going from 75% to 50% RH removes about 5 g per cubic metre. If a room stays damp, the moisture is not in the air; it is coming from somewhere, continuously. That is why the continuous term dominates and why an oversized machine does not fix a damp room any faster.
The guide range is deliberately conservative because it has to cover the worst plausible case for a floor area — poor construction, high infiltration, a drying screed, laundry, an unsealed floor slab. The psychrometric figure is what your stated conditions actually demand. Use the guide when you do not know the moisture sources; use the psychrometric method when you do.
The catch: rated capacity is not capacity in your room
A dehumidifier's advertised litres-per-day figure is measured at a specified test condition, and refrigerant machines lose output sharply as air gets colder and drier.
Which test standard matters. Before 2019 the US DOE rated at 80 °F / 60% RH (26.7 °C). The current standard rates at 65 °F / 60% RH (18.3 °C), which is more realistic and gives a lower number for the same machine — typically 20–25% lower. European products are commonly quoted at 30 °C / 80% RH, which flatters them further. A "20 L/day" European unit and a "20 L/day" current-DOE unit are not the same machine. Always check the stated test condition before comparing.
Cold spaces. A refrigerant dehumidifier in a 10–15 °C garage, cellar or unheated store may deliver a third of its nameplate figure, and below roughly 15 °C the coil begins to frost, so the machine spends part of its cycle defrosting rather than drying. Below about 10 °C a desiccant dehumidifier is usually the right technology — its output is far less temperature-sensitive, though it consumes more energy per litre at warm conditions.
Assumptions and where this stops being valid
It sizes for moisture removal, not for a problem. Persistent damp usually has a source: rising damp, a failed DPC, penetrating damp, a leak, or condensation on cold bridges. A dehumidifier manages symptoms. If the continuous load is large and unexplained, find the source first.
Steady, well-mixed air is assumed. One machine does not dry a suite of rooms through closed doors. Size and site per enclosed space, or provide air movement.
Infiltration is an estimate. The 0.5 ACH default is typical for a reasonably tight modern room. An old building with open flues can exceed 1.5 ACH, which more than doubles the continuous load. If the result is marginal, this is the input worth measuring rather than guessing.
Construction drying is a different job. Drying out a new screed or after a flood releases far more moisture than occupied-space sizing assumes, and is normally handled with hired high-capacity equipment sized on the wet material mass, not the room volume.
No condensation-risk check. Reaching a target RH does not guarantee surfaces stay above dew point. Cold bridges can still condense at 50% RH if their surface temperature is low enough.
Standards and what to do next
Rating and test conditions: US DOE 10 CFR 430 Subpart B Appendix X1 (the 65 °F basis) and the ENERGY STAR programme requirements for dehumidifiers, which is also the source of the capacity-by-area guide table. IEC 62512 covers performance measurement for household units.
Indoor humidity targets: ASHRAE Standard 55 for thermal comfort; ASHRAE 62.1/62.2 for ventilation. 30–60% RH is the usual comfort and health band. Below 40% reduces dust-mite populations; above 60% supports mould growth on susceptible surfaces.
Next steps: log temperature and humidity in the space for a week before buying — a cheap data logger will show whether the problem is continuous or driven by a daily event such as showering or drying laundry. Check the machine's rated condition against your actual room temperature and derate accordingly. Confirm the drainage arrangement: a unit sized at 3.6 L/day will fill a 2 L tank roughly every 13 hours, so continuous drainage is worth having wherever a fall to a drain exists.
Background & method
The guide mode follows the capacity ranges published by ENERGY STAR for damp, very damp and wet spaces. Psychrometric mode adds a physically based moisture balance for users who know indoor/outdoor conditions. The psychrometric calculation assumes standard atmospheric pressure of 101.325 kPa; high-altitude applications should use a pressure-adjusted model.
FAQ
Water-removal capacity falls as air temperature drops because the coil has less moisture available to condense and frosting can limit operation.
Around 45–55% RH is a common comfort and moisture-control range. Very low RH is usually unnecessary and may increase energy use.
It estimates water contained in the room air, infiltration moisture and internal moisture generation instead of relying only on floor area and dampness category.